不是主语。这句话是主系表结构，主语是mooncake，is是系动词，后面接表语，is a delicious,round cake构成复合谓语。

小学英语绘本mum's new hat

I'm wangNan,a student of No.1 Middle School. I have a gooa friend,Jim.He' an USA girl.We are both in Class Two,Grade Eight.She is 15 years old,one year older than me. We both study hard.I'm good at physices,but she's good at math.We often help each other with lessons.Both of us like helping others. We both like sports very much.Jim enjoy playing basketball.but Ilike palying soccer. She likes green but Ilike red. Tough we are different in my ways,we are good friends